The invention relates to electronic semiconductor devices, and, more particularly, to ferroelectric structures and fabrication methods for such structures.
Ferroelectric materials have applications both as capacitor dielectrics for integrated circuit memories (both non-volatile and dynamic RAM) and as uncooled infrared imagers (focal plane arrays, night vision). Thin film ceramic ferroelectrics have the greatest utility, and various compounds have been investigated, such as barium strontium titanate (BST), lead zirconate titanate (PZT), and strontium bismuth tantalate (SBT).
For non-volatile memory applications the ferroelectric should have resistance to fatigue due to polarization switching, and a large remnant polarization helps. For dynamic memory applications the ferroelectric (typically operated as a paraelectric) should have low leakage current and a large dielectric constant. For uncooled infrared imaging applications the ferroelectric should have a large pyroelectric coefficient and low dissipation loss and low dielectric constant.
FIGS. 1a-1c illustrate schematically a small array of cells/pixels of a dynamic RAM, a non-volatile RAM, and a uncooled infrared focal plane. In a DRAM the ferroelectric appears as a common capacitor dielectric (along with a common xc2xd Vdd electrode) for all capacitors of the array with the capacitors buried below metal interconnects. In a non-volatile RAM the ferroelectric only appears in the capacitors, and each row of cells has a common drive line; again the capacitors are buried below metal interconnects. In contrast, the ferroelectric uncooled focal plane capacitors abut infrared absorbers (or are themselves the absorbers) which are exposed to incoming (through an optical system) infrared radiation, and the capacitors are thermally isolated from the underlying read-out integrated circuit.
The current ferroelectric materials have problems of difficult manufacture and limited performance.
The present invention provides ferroelectric materials derived from lead titanate with substitution for lead by calcium and substitution for titanium by tin together with optional dopant additions. Preferred embodiments use metal organic decomposition (MOD) to form thin film of these ferroelectrics at low temperatures which permit use as part of integrated circuit uncooled imagers and memories.